The present invention relates to a system for monitoring containers with ultrasonic seals. It is used in the monitoring of containers containing dangerous products, precious materials, documents, etc.
The principle of monitoring by ultrasonic seals is known. It consists of fixing in a final manner an ultrasonic seal to a container to be monitored, said seal being provided with a unique, non-reproducible marking located in the vicinity of the pre-established breaking zone of the seal. This marking is obtained by structural irregularities of the matrix containing the seal, e.g. by material inclusions or cavities distributed in a random manner.
Generally, an ultrasonic seal comprises a transducer able to convert electrical excitation into an ultrasonic wave and vice versa. The structural irregularities on the path of the transmitted ultrasonic wave give rise to ultrasonic echoes detected by the transducer, which on return supply an electrical signal whose complex form is specific to the seal. Electronic means are provided for exciting the transducer and for collecting the electrical signal supplied by it, as well as for analysing said signal. In such a device, the ultrasonic transducer functions both as a transmitter and as a receiver, so that a seal only has one transducer.
In a prior art ultrasonic seal, the transducer receives through a supply cable a high voltage electric pulse, e.g. of 100 to 200 V, with a width close to 1O microseconds and a rise time of 50 to 100 ns. The transducer produces a sound wave, which is propogated through a delay line and partly reflected by the faults which it encounters. The wave reflected by these faults reaches the receiving transducer, which then supplies a low voltage signal. Its frequency is approximately 5 MHz and its peak value 1OO mV, with a delay of 5 .mu.s and a duration of 1O .mu.s. This signal forms the identity of the seal. It also contains the integrity information, bearing in mind that any significant deterioration to the seal and in particular the breaking thereof leads to a significant modification in the shape or form of the signal.
The signal received is rectified and then sampled over a few values (roughly 10 values). The values of these few samples are then digitally converted and transmitted to a processing module. The latter compares the values received (after possible digital processing) with reference values contained in a memory. It deduces therefrom the identity of the seal and checks its integrity. The processing module can initiate special procedures, such as alarms, as a function of the results of the control.
These systems are not suitable for the application envisaged by the invention, which is that of controlling a large number of containers at various times which can be widely spaced (e.g. by one year), whereby said control or inspection is to be carried out by different equipment (containers transported from one site to another).
The problem which mainly occurs in such an application is that of stability and reproducibility of the means involved for recording the "signature" of a seal, or in other words for acquiring information. Thus, if it is wished to be able to check and identify one from among many containers after an interval of one year, it is necessary that the acquisition means used are free from instabilities, fluctuations and variations. Conversely, if it is wished to be able to check the same seal with different equipment, it is necessary that the structure of the equipment is reproducible and that the signature of the seal recorded with such equipment is the same in the case of an equipment change.
The prior art systems are not suitable for this application for the following reasons. Firstly the exciting pulse, which is very narrow, can be subject to slight duration variations over a period of time, which have considerable significance on the shape of the signal supplied by the seal. This sensitivity to the width of the exciting signal is understandable if it is borne in mind that each front of the exciting pulse (front edge and then back edge) disturbs the acoustic medium in the ultrasonic seal the total signal received resulting from the interference between the two thus produced acoustic components.
Moreover and as indicated hereinbefore, there is a considerable disparity between the amplitude of the exciting signal (100 V) and that of the signal received (1OO mV), so that effective electronic means must be positioned between the receiver and the seal to prevent the former being blinded at the time of transmission. However, such means are fatally effected by drift over a period of time.
Finally, the transmitter is connected to the transmitting transducer by a cable and there again variations can appear (mismatching, interference), which are prejudicial to the reproducibility of the measurement.